1. Field of the Invention
The present invention relates to an image processing apparatus used for recording, such as a laser beam printer, an ink jet printer etc., particularly an image processing apparatus which processes multi-valued images.
2. Related Background Art
In recent years, a recording apparatus employing electronic photography system such as a laser beam printer has come to be used widely as the output apparatus of the computer. These apparatus have many advantages such as high picture quality, low noise, etc. and particularly because of its high picture quality, it has constituted one of the factors contributing to the rapid expansion of the field of desk top publishing (DTP).
Laser beam printer 100 is composed of printer controller 101 and printer engine 102 as shown in FIG. 16. Printer controller 101 decomposes the code data transmitted from the host computer 103 into dot data, develops them on the bit map memory (not shown in the drawing) and transmits them to printer engine unit 102. Since the said bit map memory is binary memory which works on the principle of printing dot or not, for printing of the image such as the photograph of the laser beam printer they have widely used such method as to binarize the multi-valued signal by dither method etc. at host 103 side in order to obtain the desired gradation and then print them.
On the other hand, in recent years such method has also been proposed wherein printer controller 101 is provided with the multi-valued bit map memory, the multi-valued data developed on such multi-valued memory are transmitted to the printer engine unit 102, binarization processing such as density pattern processing, pulse width modulation of recorded pixel etc. is performed at the printer engine unit 102 and thus intermediate tone is printed out.
When a laser beam printer is taken for example, it is known that a preferred quality of image is obtained if plural number of dots are grouped into one multi-valued pixel and density of such multi-valued pixel is modulated into about 64 gradations, because of the characteristics of electronic photographing process.
However, in the aforesaid conventional method, when the printer controller 101 has the bit map memory of 6 bit with the resolution of printer engine unit 102, the memory capacity becomes 6 times that of the bit map memory of binary data and the apparatus becomes inevitably costly. It is also possible to have separately the bit map memory of 6 bit with the resolution of the said multi-valued pixel which groups plural number of dots and the bit map memory of one bit with the resolution of printer engine but in such case processing of image data is complicated and besides, due to the difference in resolution between binary data and multi-valued data, problems occur at the boundary of the two data. To prevent such problem, it is necessary to provide the binary/multi-value switching signal with the resolution of binary data and it accompanies the drawback to result in the increase of memory capacity.
When one bit is treated in multi-value for example in about 4 bit and such dots are coupled at controller 101 side, it is possible to obtain gradation while maintaining a relatively high resolution but optimization of the pattern of such coupling is difficult unless one has deep knowledge about the characteristics of the electronic photographing process of the printer.